Modelling of Electric Arc Welding: arc-electrode coupling

96 Pages·2013·1.69 MB·English

THESIS FOR THE DEGREE OF LICENTIATE OF ENGINEERING
Modelling of Electric Arc Welding:
arc-electrode coupling
ALIREZA JAVIDI SHIRVAN
DepartmentofAppliedMechanics
DivisionofFluidDynamics
CHALMERS UNIVERSITY OF TECHNOLOGY
Gothenburg,Sweden2013
ModellingofElectricArcWelding: arc-electrodecoupling
ALIREZA JAVIDI SHIRVAN
c ALIREZA JAVIDI SHIRVAN, 2013.
(cid:13)
Thesis for the degree of Licentiate of Engineering 2013:12
ISSN 1652-8565
Department of Applied Mechanics
Division of Fluid Dynamics
Chalmers University of Technology
SE–412 96 Gothenburg
Sweden
Telephone + 46 (0)31 – 772 1000
Typeset by the author using LATEX.
Chalmers Reproservice
Gothenburg, Sweden 2013
ModellingofElectricArcWelding: arc-electrodecoupling
ThesisforthedegreeofLicentiateofEngineeringinThermoandFluidDynamics
ALIREZAJAVIDISHIRVAN
AppliedMechanics
DivisionofFluidDynamics
ChalmersUniversityofTechnology
ABSTRACT
Arc welding still requires deeper process understanding and more accurate
prediction of the heat transferred to the base metal. This can be provided by
CFDmodelling.
Most works done to model arc discharge using CFD consider the arc core
alone. Arc core simulation requires applying extrapolated experimental data as
boundary conditions on the electrodes. This limits the applicability. To become
independent of experimental input the electrodes need to be included in the arc
model. The most critical part is then the interface layer between the electrodes
andthearccore. Thisinterfaceiscomplexandnon-uniform,withspecificphys-
icalphenomena.
The presentwork reviewsthe concepts ofplasma andarc discharges thatare
useful for this problem. The main sub-regions of the model are described, and
theirdominantphysicalrolesarediscussed.
The coupled arc-electrode model is developed in different steps. First cou-
pling solid and fluid regions for a simpler problem without complex coupling
interface. This is applied to a laser welding problem using the CFD software
OpenFOAM. The second step is the modelling of the interface layer between
cathode and arc, or cathode layer. Different modelling approaches available in
the literature are studied to determine their advantages and drawbacks. One of
them developed by Cayla is used and further improved so as to satisfy the ba-
sic principles of charge and energy conservation in the different regions of the
cathode layer. A numerical procedure is presented. The model, implemented
in MATLAB, is tested for different arc core and cathode conditions. The main
characteristics calculated with the interface layer model are in good agreement
with the reference literature. The future step will be the implementation of the
interfacelayermodelinOpenFOAM.
Keywords: arcweldingsimulation,plasma,arcdischarge,cathodelayer,sheath
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Acknowledgments
First I would like to express my sincere thanks to my main supervisor associate
professorIsabelleChoquetforherhugesupportandencouragement. Thankyou
forbelievinginmeandguidingmewithpatiencetofindmywayintheresearch
field. It is a privilege being your student and I wish I would keep learning from
you.
Iwouldliketothankmyco-supervisorassociateprofessorHåkanNilssonforthe
support and valuable courses, discussions and feedbacks regarding the project
andmorespecificallytheOpenFOAMsoftware.
I do appreciate all comments and discussions by my colleagues in the Weld-
inggroupofProductionTechnologyWestatUniversityWestinTrollhättan.
The financial support for this work provided by IV-Mechanical Engineering de-
partmentatUniversityWestisacknowledged.
I finally give thanks to my loving wife Salma for her endless kindness and great
supportandlove.
ALIREZAJAVIDISHIRVAN
Gothenburg,May2013
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Preface
Thisthesisconsistsofanextendedsummaryandthefollowingpaper:
Paper A
Alireza Javidi Shirvan, Isabelle Choquet, and Håkan Nilsson. Nu-
merical modelling of shielding gas flow and heat transfer in laser
welding process. In Proceedings of The 5th International Swedish
ProductionSymposium,Linköping,Sweden,pages269–276,2012.
Otherpublicationsrelatedtothethesisbytheauthor
Publication I (attached in Appendix A)
Isabelle Choquet, Alireza Javidi Shirvan, and Håkan Nilsson. Elec-
tric welding arc modeling with the solver OpenFOAM - a compari-
son of different electromagnetic models. In International Institute
ofWeldingDocumentNo212-1189-11, 2011.
Publication II
Isabelle Choquet, Alireza Javidi Shirvan, and Håkan Nilsson. On
thechoiceofelectromagneticmodelforshorthigh-intensityarcs,ap-
pliedtowelding. JournalofPhysicsD:AppliedPhysics,45(20):205203,
2012.
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Contents
Acknowledgments iii
Contents vii
1 Introduction 5
1.1 Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2 Electricdischarges . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3 Outlineofthiswork . . . . . . . . . . . . . . . . . . . . . . . . 10
2 Physicsofelectricarcsappliedtowelding 15
2.1 LTEPlasmacore . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2 Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3 Spacechargelayer(cathodesheath) . . . . . . . . . . . . . . . 24
2.4 Ionizationzone(cathodepre-sheath) . . . . . . . . . . . . . . . 25
3 Solid-fluidcoupling-summaryofPaperA 29
3.1 Governingequations . . . . . . . . . . . . . . . . . . . . . . . 30
3.1.1 Governingequationsinthefluidpart . . . . . . . . . . . 31
3.1.2 Governingequationinthesolidpart . . . . . . . . . . . 31
3.2 Testcasesandnumericalsettings . . . . . . . . . . . . . . . . . 32
3.3 Boundaryconditions . . . . . . . . . . . . . . . . . . . . . . . 34
3.4 Temperaturedistribution . . . . . . . . . . . . . . . . . . . . . 35
3.5 Velocitydistribution . . . . . . . . . . . . . . . . . . . . . . . . 37
3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4 Cathodelayer 41
4.1 Problemdescription . . . . . . . . . . . . . . . . . . . . . . . . 41
4.2 Existingmodels . . . . . . . . . . . . . . . . . . . . . . . . . . 42
5 Cathodelayer-model 47
5.1 Currentdensitiesinthecathodelayer . . . . . . . . . . . . . . . 48
5.2 Numberdensitiesatthesheath/pre-sheathinterface . . . . . . . 55
5.2.1 Sahaequationsbasedontheelectrontemperature . . . . 55
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CONTENTS
5.2.2 Dalton’slaw . . . . . . . . . . . . . . . . . . . . . . . 56
5.2.3 Electricneutrality . . . . . . . . . . . . . . . . . . . . . 57
5.3 Temperaturesandsheathvoltagedrop . . . . . . . . . . . . . . 57
5.3.1 Energybalanceatthesheath/cathodeinterface . . . . . 58
5.3.2 Energybalanceinthepre-sheath . . . . . . . . . . . . . 62
5.3.3 Currentconservation . . . . . . . . . . . . . . . . . . . 65
5.4 Calculationprocedure . . . . . . . . . . . . . . . . . . . . . . . 65
5.5 Numericalprocedure . . . . . . . . . . . . . . . . . . . . . . . 66
6 Cathodelayer-results 69
7 Conclusionandperspective 79
References 81
AppendixA 87
PaperA 105
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